In this case, the defendant was in a car when an officer came up and arrested him for being within 500 feet of a school while being a child sex offender. His defense was that he dropped off a child relative to deliver an item and was waiting for him to come back when the officer approached.
The defendant challenged the constitutionality of the statute on vagueness grounds, which the Illinois Supreme Court found insufficient.
The case is People v. Archie Howard.
http://www.illinoiscourts.gov/Opinions/SupremeCourt/2017/120443.pdf